Can the assignment block assign a variable sized-signal to a fixed-sized signal?

I am wishing to do a bunch of operations on a signal. The source of the signal is variable-sized and so the variable-size propogates throughout the model. I wish to assign a variable sized-siganl (512x2102) to a zero matrix of the same size. An error appears showing that the dimensions are not equal, however they are equal.

Is this problem due to the U port being variable sized?

[Scott MacKenzie]

It would help if you included your code and the complete error message generated.